Field of Invention
The present invention relates to a technical field of gas chromatography, and more particularly to a wavy micro gas chromatography column.
Description of Related Arts
With the social development, the industrialization process, and the continuously rising living standard, the environmental pollution causes more and more problems. The emergency monitoring requires the environmental monitoring department to accurately identify the type, the concentration, the toxicity, and the distribution of the pollutant as quickly as possible. The portable gas chromatograph has the advantages of small volume, light weight, battery power supply and on-site operation. It can meet the requirements of different analysis environments and is generally able to qualitatively analyze the pollutant and obtain a preliminary quantitative result in minutes. Thus, developing a portable gas chromatograph having the high sensitivity, the high resolution, and the high cost performance has become an important research direction in the chromatography field.
Although the conventional gas chromatograph has the strong mixture separation ability, the conventional gas chromatograph has the problems of large volume, heavy weight, long separation time, and relatively high energy consumption. The micromachining technology is able to effectively realize the miniaturization of the gas chromatograph which accelerates the separation speed, consumes less power and becomes more portable.
Like the conventional chromatography column, the main performance index to examine the micro-electro-mechanical system (MEMS) chromatography column is the separation efficiency. Besides the factor of the stationary phase, the separation efficiency is also influenced by the geometric structure of the chromatography column. The main influence factors comprise following three points.
(1) The Aspect Ratio
Because the conventional MEMS chromatography columns are mostly micro-machined and etched through a deep reactive-ion etching (DRIE) technology, the cross section of the chromatography groove is rectangular, which is different from the circular cross section of the conventional capillary chromatography column. For the rectangular groove, the deeper the depth and the narrower the width, namely the larger the aspect ratio, the higher separation efficiency will be obtained for the MEMS chromatography column. The aspect ratio is the main influence factor of the separation efficiency.
(2) The Plane Layout of the Groove
The chromatography groove must have a certain length (generally 0.5-6 m). Thus, in order to coil the micro chromatography groove having the enough length on the silicon wafer which has an area as small as possible, the plane configurations of the conventional micro chromatography columns are mainly serpentine and spiral (as showed in FIG. 1). The University of Illinois, USA, systematically compared the separation performances of the circular-spiral, the square-spiral and the serpentine micro chromatography columns. The result demonstrated that, no matter under the isothermal mode or temperature-programmed mode, the serpentine chromatography column has the sharpest peak and the highest resolution, referring to the non-patent literature (Radadia A D, Salehi-Khojin A, Masel R I, et al. The effect of microcolumn geometry on the performance of micro-gas chromatography columns for chip scale gas analyzers, Sens Actuators B, 2010, 150: 456-464).
(3) The Microstructure in the Groove
The above-described rectangular groove of the MEMS chromatography column is completely hollow and called open tubular column. When the rectangular groove has a microstructure having finer size (such as the micro posts showed in FIG. 2) micro-machined therein, the groove is called semi-packed column. Compared with the open tubular column having the same groove size, the semi-packed chromatography column has the higher separation efficiency, because the airflow in the semi-packed column is more uniform than in the open tubular column. The Virginia Polytechnic Institute and State University analyzed the airflow in the open tubular column and the semi-packed column by a finite element analysis software, and the result thereof is showed in FIG. 9 of the non-patent literature (Ali S, Ashraf-Khorassanib M, Taylor L T, et al. MEMS-based semi-packed gas chromatography columns, Sens Actuators B, 2009, 141: 309-315). The figure is a color drawing, wherein the carrier gas velocity in the red area is fastest and in the blue area is slowest. Thus, for the open tubular column, the airflow at the center is fastest and at the edge is slowest, leading to a laminated flow distribution for a deep vertical groove, which is disadvantageous for the contact between the gas molecules and the stationary phase. In comparison, the laminar flow effect of the gas in the semi-packed column is weakened, which increases the separation efficiency of the chromatography column. Thus, the flow distribution in the groove of the micro chromatography column has a great effect on the separation performance of the chromatography column.
Although the semi-packed column has the higher separation efficiency than the open tubular column, the MEMS machining of the semi-packed column is more difficult since the much smaller size and finer substructures are needed to be realized.